Impression cylinder construction to prevent streaking in letterpress



L. KIRKUS ETAL 3,395,638 IMPRESSION CYLINDER CONSTRUCTION TO PREVENT Aug. 6, 1968 STREAKING IN LETTERPRESS Filed Aug. 13, 1965 2 Sheets-Sheet 1 w Z v b. Eh R E 4 PD 5 m b N Gs 3 E II 5 V M m M u W M L N 2 .0 C

5: ROTATION Aug. 6, 1968 KlRKUs ETAL 3,395,538

IMPRESSION CYLINDER CONSTRUCTION TO PREVENT STREAKING IN LETTERPRESS Filed Aug. 13, 1965 2 Sheets-Sheet 2 NO 54 CONTACT EXGERATED) fi'v&?s

Kemws Luis TYMA United States Patent 6 3,395,638 IMPRESSION CYLINDER CONSTRUCTION TO PREVENT STREAKING IN LE'ITERPRESS Ludwig Kirkus, Chicago, and Louis S. Tyma, Hinsdale,

Ill., assignors to Miehle-Goss-Dexter, Incorporated, Chicago, 111., a corporation of Delaware Filed Aug. 13, 1965, Ser. No. 479,423 7 Claims. (Cl. 101-216) ABSTRACT OF THE DISCLOSURE A printing couple for a letterpress in which the impression cylinder is of non-circular contour having a relief in the form of a flattened strip extending longitudinally thereon tapering to zero thickness at its lateral edges which is so phased as to be generally centered with respect to the gutter between adjacent plates and which has a width which is substantially greater than the Width of the gutter to provide a more gradual buildup and decay of pressure at the leading and trailing edges of the plate. In one embodiment the relief is in the form of a shallow, axially extending groove formed in an underlayment.

In the operation of newspaper letterpresses at modern day speeds, there is a problem of streaking in the printing which is particularly noticeable where there are large solidly black or nearly black areas at the leading edge portion of the printing plate. This streaking is evidenced by alternating light and dark stripes which are parallel to the cylinder axis and which extend across the printed page at equally spaced intervals, usually within the range of about one inch to several inches, the spacing and intensity varying with press speed. Conventional stereotype printing plates are approximately /2" thick and, when installed on the plate cylinder, are separated by a gap or gutter, varying from /2" to 2" in radial extent, corresponding to the non-printed margin between pages. One explanation of the streaking is that the plate and impression cylinders comprising a printing couple vibrate transversely during rotation as they roll over the gaps between the plates, because the impression force, which is required for uniform printing over the plate area, is suddenly relieved and reapplied.

Notwithstanding the mass and apparent rigidity of the usual plate and impression cylinders, it is found that there is sufiicient resiliency, or spring, in the cylinders and their mountings that the cylinders move relatively toward and away from one another, with the worst streaking resulting when the movement is so great that actual separation occurs. We have found that the effect of the sudden release of the impression force at the plate gap and the sudden increase or buildup of the force when the edge of the succeeding plate engages the impression cylinder surface after the gap is passed, may be cumulative, providing maximum streaking when the resulting vibrations are in phase.

It is, therefore, an object of the invention to provide means for reducing streaking in a printing operation which is more effective than the previously attempted solutions and which has been found to substantially reduce or eliminate streaking at the operating speed of the press while avoiding any complications of critical speed resonance, such as those occurring where springs are used. It is an object, in addition, to provide means for alleviating streaking which is particularly effective at a desired operating or production speed of the press but the benefits of which are not limited to a particular speed but extend over a relatively wide speed range. It is another object to provide an impression cylinder which is of non-circular contour to reduce impact and sudden release at the edges of the printing areas during the rotative cycle. It is more specifically an object of the present invention to provide an impression cylinder which is progressively relieved at the regions where it encounters the gap or gutter between plates thereby to avoid the sudden change in load which occurs as the leading and trailing edges of adjacent printing areas engage or separate from the impression cylinder which gives rise to vibration of the cylinders toward and away from each other.

It is a further object to provide a simplified design procedure for establishing a near optimum anti-streak condition in an existing design of press and which automatically takes into account such parameters as desired operating speed and the natural period of cylinder vibration. Thus, it is an object to provide an improved printing couple in which the anti-streak feature is obtained at low cost and with only minor modification of the impression cylinder permitting presses in the field to be easily and quickly modified to secure the benefits of the invention.

It is still another object to achieve the foregoing benefits while enabling standard blanket material to be used as a covering for the impression cylinder.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIGURE 1 is a fragmentary end view of a printing couple including a plate cylinder and cooperating impression cylinder embodying the present invention.

FIG. 2 is a fragmentary view showing the relationship between the cylinders at the gutter region enlarged for clarity.

FIG. 2a is a diagram similar to FIG. 2., with the curvatures purposely exaggerated and with the blanket omitted for simplicity.

FIG. 3a is a plot showing variation of impression force between cylinders when impression cylinder is round and when relief has been formed on the surface of the impression cylinder under static conditions, i.e., with the plate cylinder rotated very slowly.

FIG. 3b is a plot similar to FIG. 3a: but showing a force variation characteristic of actual running conditions.

FIG. 4 is a view similar to FIG. 2 but showing use of an underlayment to create relief.

FIG. 5 shows the relief or groove formed in the underlayment when the latter is in flat condition, before it is attached to the impression cylinder.

FIG. 6 is a fragmentary view of a two-page or tabloid printing plate showing the gutter at the margin position between pages.

While the invention will be described in connection with certain preferred embodiments, it will be understood that we do not intend to be limited to the embodiments shown but intend to cover the various alternative and equivalent embodiments which may be included within the scope and spirit of the appended claims.

Turning now to the drawings, FIGURE 1 shows a plate cylinder 10 of a newspaper type rotary letterpress having a shaft 11 mounted in the press frame fragmentarily shown at 12. Secured to the surface of the plate cylinder are stereotype printing plates 21, 22, the printing areas of which are separated by a gap or gutter space 25 (FIG. 2) and which are securely held in place on the cylinder by lockup mechanisms 23, 24. The plates, as is characteristic of stereotype plates, are formed by the usual casting process and are rigid and relatively thick. Similar structure will be found at one or more additional positions about the cylinder, depending upon the diameter of the cylinder and the size of the printed page. While the invention is not limited to use of any particular type of plate lockup mechanism, we prefer to use a mechanism which employs fingers to engage pockets milled in the underside of the plates adjacent the straight edges. This type of lockup, usually referred to as a tension lockup is well known in the art and, for the details of construction, reference may be made to prior Chase and Manning Patent 2,639,668 which issued May 26, 1953.

Cooperating with the plate cylinder and forming a printing couple therewith for printing on one side of web 36, is an impression cylinder 30 having the same nominal working diameter as the plate cylinder with plates installed. The cylinder 30 is made from a solid forging and is supported by a shaft 31 journaled in a movable mount 32 in the frame 12, with adjusting means for changing the center-to-center distance between the two cylinders to obtain the impression force required for printing. It will be understood that conventional high quality printing press bearings, characterized by absence of play, are used for journalling the shafts 11, 31. The adjusting means indicated diagrammatically at 33 may be conventional. The impression cylinder 30 is covered with a conventional blanket 35 made of yieldable and slightly resilient material serving as a backup for web 36 to insure all-over contact between the web and the type face on the plate notwithstanding minor variations in the height of the type face. To secure the printing contact, the impression cylinder is adjustably advanced toward the plate cylinder, beyond the point of barely touching between the type face and the web, squeezing the plate and blanket together by an additional increment of distance, which may be referred to as the impression advancement, thus creating pressure loading between the cylinders. It will be understood that the impression advancement may be increased to the point necessary to secure impression even at any low spots which may exist on the plate.

For the purpose of rotating the two cylinders in unison with one another the usual press drive is provided, indicated diagrammatically at 40, having driving connections 41, 42 to the two cylinders which include, in a practical case, engaging gears having a pitch diameter the radius of which corresponds to the radius of the working surfaces indicated at R in the drawings. As the cylinders rotate in unison, the same point on the periphery of the impression cylinder will always coincide with the center of the gutter 25 during each revolution, and this relation, indicated at 45 in FIG. 1, may be referred to as the gutter position of the cylinders. When the cylinders are in the relation shown in FIG. 1, the plates are not in contact with the blanket and there is no impression so the load is zero. However, the load will suddenly increase to the running value, or above, when a plate engages the surface of the impression cylinder and upon rotation of the cylinders.

In accordance with the present invention, it has been found that streaking effects may be minimized by making the impression cylinder of non-circular configuration to produce a gradual change of the impression force between the impression cylinder and the printing, or land, surfaces at the edges of the printing plates. More specifically in accordance with the invention the impression cylinder is formed, by removing an axially extending strip of material from the surface of the cylinder which is of crescent-like cross section, with a relief which extends overlappingly beyond the edges of the printing area on each side of the gutter with the result that the impression force between the cylinders undergoes a more gradual transition from zero to a running value at the plate leading edge and from the running value back to zero at the trailing edge.

Thus, referring particularly to FIGS. 2 and 2a, a relief segment is provided, defined by a surface 50 on the impression cylinder which is centered at the gutter position 45, and having a total width w extending from a first edge 51 to a second edge 52 sufiicient to overlap the land area along the edges 53, 54 of the printing plates. The relieved surface 50, as is brought out in FIG. 2a exaggerated for clarity, falls within the space defined by the normal cylindrical outer surface 55 of the impression cylinder and a chordal or flat surface indicated at 56. This results in a relief segment of crescent shaped cross section as indicated by the cross hatched area 57 in FIG. 2a. The term crescent shaped relief segment shall therefore be understood to mean a relief in the shape of a generally flattened, longitudinally extending strip having a cross section defined by an outer edge corresponding to the nominal radius of the cylinder with a central portion of maximum thickness tapering smoothly to zero thickness along the lateral edges. As shown, the radius of curvature of the relieved surface 50 exceeds the nominal radius R of the impression cylinder and need not be constant.

It will be understood that the invention is not limited to any fixed amount of relief because of widely varying conditions which are found in press operations. However, the amount of relief, with the edge of the printing area on the plate on dead center, should be less than the impression advancement between the two cylinders, referred to above, in order to insure that printing contact with the web is maintained all the way out to the edge of the printing area especially at low speed. Thus, stated in general terms, the depth of relief at its center portion should be maximized short of losing impression contact with the printing area at the edges of the plates. In any event, the plate will first contact the impression cylinder at a point in the region of the relief and, therefore, the resulting pressure loading will be less than the loading when the plate is in contact with the non-relieved portion of the impression cylinder.

The beneficial result brought about by the present invention may be readily visualized by a graphic representation of the variation of the impression force between the cylinders of a conventional press as the plate gap passes dead center under the conditions of slow speed and running speed compared to the impression force variations where the present invention is employed. Thus, in FIG. 3a there is shown, in solid line, a curve 60 which shows the general nature of the change in force when relief is provided under slow speed or what may be termed static conditions when single page semi-cylindrical plates are used. When the plate gap is on the dead center line as shown in FIG. 2, the two cylinders are out of engagement so the load between them is zero. As the cylinders are rotated slowly, the leading edge of the printing area on the plate 21 engages the impression cylinder as indicated at 61 and the force rises progressively to point 63 which represents the force when the edge of the plate is at dead center. Upon further rotation the force rises with a flatter characteristic as indicated until a point .65 is reached where the edge of the relief is on dead center, following which the force is constant as shown by horizontal line 66. At the other end of the plate the converse occurs. After the second edge of the succeeding relief on the impression cylinder arrives at dead center position, indicated at 67, the force decreases to a point 68 when the trailing edge 54 of the plate is on dead center, and this is followed by a decrease to zero when the trailing edge of the plate leaves the surface of the blanket on the impression cylinder at 69. The above is to be contrasted with the force condition in conventional presses indicated by the dotted line 70. The force, starting from zero, begins to build up at an earlier point 71 as the leading edge of the printing area engages the impression cylinder, rising steeply to maximum at 73 as the edge of the plate moves to dead center. Following this the impression force remains constant during further rotation until it falls abruptly to zero when the opposite edge of the plate passes dead center. It is apparent from FIG. 3a that the initial portion and the final portion of the curve 60 are both of substantially reduced average slope indicating a more gradual transition of impression force from zero to maximum at the leading edge of the plate and from maximum to zero at the trailing edge of the plate.

The importance of this change under actual running conditions may be appreciated upon reference to FIG. 3b where the solid line again represents the force variation characteristic of the invention while the dotted line represents the force variation encountered in prior art constructions. Thus, as shown by the curve 60a the force rises progressively to its maximum value 65a, any breakpoint, or abrupt change in rate, being practically eliminated because of the inertia of the members under running conditions. After the maximum force 65a is reached, the force decreases to a value somewhat less than the running value, as indicated at 66a, soon stabilizing at the running value at 66. At the end of the plate the force drops off smoothly as indicated at 67a, with little or no tendency toward a negative overtravel.

By contrast in a conventional press characterized by the dotted curve 70a, the force rises steeply from the point of cantact 71a as the printing edge of the plate engages the impression cylinder with a sudden blow, the impact causing the force momentarily to soar to a value 75a, which is much higher than the running value. This is followed by a rapid reactionary decrease almost to zero, as indicated at 76a. Because of the resilience of the system, a number of cycles of variation occur before the force stabilizes to running value at v66. It is the extreme values at 75a and 76a of the force and the resulting movement of the cylinders which account for the dark and light streaks in the product of a conventional press. The situation may be worsened by the fact that at the trailing edge of the plate a sudden decrease of the force occurs at 77a, resulting in a negative overtravel at the gutter position, shown by the portion 79a of the curve. At certain critical speeds, depending on the gutter width, it is possible for the rebound from the negative force to be in phase with the rapid increase of force caused by the sudden blow as the leading edge of the succeeding plate engages the cylinders so that peak forces may be even greater than those due to the impact effect alone.

It should be pointed out in connection with FIGS. 30 and 312 that these figures are only intended to show the characteristic shapes of the force cuves and are not intended to denote actual force values which may vary from press to press, depending upon operating conditions.

In accordance with one embodiment of the invention, an underlayment may be provided between the blanket and the impression cylinder to provide the relief and thus create a condition analogous to a relief on the surface of the cylinder. The underlayment is formed of a sheet of hard material which has a thinned portion which forms the relief. The underlayment is placed on the surface of the impression cylinder 30a so that the thin portion extends axially and is centered in phase with the gutter, the thinned portions being sufficiently wide, peripherally, so as to overlap the edges of the printing areas of the adjacent plates. Thus, referring to FIG. 4, which is similar to FIG. 2, with corresponding elements being indicated by corresponding reference numerals plus subscript a, the impression cylinder 30a is provided with an underlayment 80, below the blanket, formed from a plate of metal having a thin portion 50a extending peripherally between points 51a, 52a and which runs axially of the cylinder. In a practical case, the underlayment may be formed of a sheet of metal having a thickness of 0.008 with the relief 50a being machined therein either while the sheet is flat or while it is drawn about a cylindrical form. Turning to FIG. 5, a portion of the underlayment is shown in the flat condition. Here it will be noted that the relief 50a is formed of three portions, a root portion 81 which may have a thickness of 0.003", and adjacent ramp portions 82, 83 which merge with the surface 84 of the underlayment. The total width of the relief segment formed by surface 5001 may be on the same order as that of the width w of the segment previously referred to, to provide the necessary overlap of the printing area of the plates. Use of underlayment is particularly desirable in employing the present invention in an existing press, since it does not require remachining of the impression cylinder itself to form the relief.

If desired to make the relief merge more smoothly with the running surface of the impression cylinder, the edges indicated at 51 and 52 in FIG. 2 and 51a, 52a in FIG. 4 may be slightly rounded.

With the above teachings in mind one skilled in the art may practice the invention without difficulty, the depth and width of relief being the maximum which can be used without loss of printing impression at the edges of the printing area of the plate. In the case of an impression cylinder of a range of diameter commonly used in a newspaper printing press, the maximum depth of relief, While preserving impression, may be in the order of 25% to 75% of the impression advancement, on the order of, say, 0.006 inch. The width of the relief may be on the order of 4.75 inches for a gutter width to 1.5 inch. Stated in terms of overlap (see FIG. 2), an overlap up to about two inches is preferred. In practicing the invention the radius of curvature of the relieved surface will exceed the nominal radius of the cylinder by 2-15 for cylinders of usual diameter, when the segment is bounded by two circular surfaces.

In one of the aspects of the present invention the width of the relief may be determined by a simple procedure which takes into account the various structural and operating parameters and which tends to minimize streaking at the production speed at which the press is to be run. The scheme is particularly useful in applying the invention to an existing press not embodying the invention. The procedure involves the step of running the press at the speed to be used for production and noting the wavelength of any streaking, i.e., the physical spacing or distance between light-to-light or dark-to-dark areas. A relief is then formed on the impression cylinder having a width which is proportional to the streak wavelength and having a depth which is such as to maintain impression to the edges of the printing areas. In a practical case the width may be twice the wavelength, but in any event within a range of 1.50 and 3.50 times the wavelength.

A still further advantage of the present impression cylinder construction, when the gutter extends from one end of the printing cylinder to the other, is that the relief provides increased clearance between the two cylinders when the gutter is on dead center position, facilitating the threading of a web between the cylinders without necessity for changing the impression adjustment or temporarily backing off one cylinder with respect to the other.

In the above description, reference has been made to only a single gutter 25. It will be understood by one skilled in the art that the invention is not limited to any particular number of gutters which may exist on the printing cylinder and relieved areas may be provided on the impression cylinder corresponding to each gutter position.

While the invention has been discussed above in connection with the gutter which exists between adjacent plates, it will be understood by one skilled in the art that the problem of streaking is particularly troublesome in the case of printing pages of tabloid size and in which each stereo printing plate may peripherally accommodate printing areas for two tabloid pages with a non-printing area in between to define a clear margin. Thus, as shown in FIG. 6, there is provided a printing cylinder 10b having a plate which is divided into two printing areas 21b, 22b separated by a shall-ow groove 25b. For the purpose of defining the present invention the groove 25b between the adjacent printing areas will be considered as a gutter, completely analogous to the gutters 25, 25a referred to in the preceding embodiments. The edges of the printing areas indicated at 53b, 54b correspond to the edges 53, 54 and 53a, 54a previously referred to.

We claim as our invention:

1. In a printing letterpress, the combination comprising a press frame, a printing couple formed of a plate cylinder and an impression cylinder supported on shafts journaled in the frame, drive means for rotating the cylinders precisely in unison, said impression cylinder having a yieldable blanket thereon and said plate cylinder having lockup means for securing to the surface thereof printing plates with the adjacent marginal edges of the printing areas peripherally spaced from one another to define a gutter as well as a corresponding gutter position on the impression cylinder, means for adjusting the distance between shafts to create pressure between the surfaces of said plate and impression cylinders, the impression cylinder having a relief segment extending longitudinally thereof at the gutter position and generally centered with respect to the gutter position, said relief segment being in the form of an elongated generally flattened strip having a central portion of maximum thickness tapering smoothly to zero thickness along its lateral edges and substantially exceeding the width of the gutter so that the pressure created between the impression cylinder and the plate cylinder is increased relatively gradu ally at the leading edge of a printing area and decreased gradually at the trailing edge as the cylinders rotate.

2. In a printing letterpress, the combination comprising a frame, a plate cylinder supported on shafts journaled in the frame, plates mounted on the surface of the plate cylinder with adjacent edges of the printing areas peripherally spaced to define a gutter bounded by land surfaces on the plates, an impression cylinder journaled in the frame adjacent the plate cylinder, said impression cylinder having a yieldable blanket and having means for adjusting the same toward the plate cylinder to a position beyond the point of contact by an impression advancement so that the plates exeit an impression force and produce an impression in the blanket but with the impression force falling to zero when said gutter is on dead center, drive means for rotating the cylinders precisely in unison so that the same surfaces on the impression cylinder always engage the land surfaces on the plates during successive revolutions of the plate cylinder and im pression cylinder, the land-engaging surfaces on the impression cylinder being relatively shaped to provide progressive relief, the relief being in the form of an elongated generally fiattened strip having a central portion of maximum thickness phased so as to be generally centered with respect to the gutter but tapering smoothly to zero thickness along its lateral edges for relatively gradual change in impression force between a running value existing over the major area of the plates and a reduced value at the edges of the printing areas adjacent the gutter thereby to reduce the sudden change of forces at the edges of the plates and the resulting transverse vibration of the cylinders as the cylinders rotate, the relief having a radial thickness at the edges of the printing areas which is less than the impression advancement.

3. In a printing letterpress, the combination comprising a frame, a plate cylinder journaled in the frame, plates mounted on the surface of the plate cylinder and having the edges of the printing areas peripherally spaced to define a gutter bounded by land surfaces on the plates, an impression cylinder journaled in the frame adjacent the plate cylinder and having a yieldable blanket mounted thereon, means for adjusting the distance between the axes of said plate and impression cylinders to create pressure between them, drive means for rotating the cylinders precisely in unison, said impression cylinder having a crescent like longitudinal relief under the blanket defined by an outer edge corresponding to the nominal radius of the impression cylinder and having a central portion of maximum thickness tapering to zero thickness at each end and in a peripheral direction of said impression cylinder, said relief being generally centered with respect to the gutter and of a peripheral width which extends overlappingly beyond the gutter to provide relatively gradual change of pressure at the land surfaces, the radius of curvature of the cylinder at the relief being from about 1.02 to about 1.15 times the nominal radius of the impression cylinder.

4. In a printing letterpress, the combination comprising a frame, a plate cylinder journaled in the frame, plates mounted on the surface of the plate cylinder and having adjacent edges of the printing areas peripherally spaced to define a gutter bounded by land surfaces on the plates, an impression cylinder journaled in the frame adjacent the plate cylinder and having a yieldable blanket for cooperating therewith, means for adjusting the distance between the cylinders to create pressure between the surfaces of said plate and impression cylinders, drive means for rotating the cylinders precisely in unison, an underlayment interposed between the blanket and the impression cylinder and extending periphrally about said impression cylinder, said underlayment formed of a sheet of relatively hard material and having at least one shallow axially extending groove formed therein merging at its edges in a taper configuration with the outer surface of the underlayment so that when the underlayment is installed upon the impression cylinder it defines a relief segment having tapered edges extending in a peripheral direction thereon which coincides with the gutter and extends overlappingly beyond the gutter to provide relief for the land surfaces at the edges of the plates thereby to provide a relatively gradual change in impression force at the edges of the plates adjacent the gutter.

5. In a printing letterpress, the combination comprising a press frame, a printing couple formed of a plate cylinder and an impression cylinder journaled in the frame, means for rotating the cylinders precisely in unison, said impression cylinder having a yieldable blanket and said plate cylinder having lockup means for securing to the surface thereof printing plates with the edges of the printing areas peripherally spaced from one another to define a gutter as well as a coincident gutter position on the impression cylinder, the impression cylinder being advanced toward the plate cylinder so that the plates press inwardly upon the blanket producing an impression therein, the impression cylinder having a relief in the shape of a segment of crescent cross section defined by an outer edge corresponding to the nominal radius of the cylinder and having a central portion of maximum thickness tapering to zero thickness of each end in a peripheral direction of said impression cylinder, said relief extending longitudinally of said impression cylinder and generally centered with respect to the said gutter position and substantially exceeding the peripheral width of the gutter, said relief segment having a depth which provides suflicient clearance between the blanket on the impression cylinder and the plates on the plate cylinder when the gutter is in its dead center position to facilitate the threading of a web about the plate cylinder without necessity for backing off the cylinders and yet being sufficiently shallow so that impression is retained in the blanket all the way out to the marginal edges adjacent the gutter.

6. The method of alleviating streaking in a printing letterpress having a cylinder with provision for mounting stereotype plates spaced thereon and a blanketed impression cylinder with impression force between them which includes the steps of running the press at the desired production speed, measuring the wavelength of streaking on the printed product, and then forming a relief on the impression cylinder at the gutter position Which overlaps the printing edges and which has a width, measured peripherally, which is between 1.50 and 3.50 times the wavelength of the observed streaking vibration.

7. In a printing letterpress, the combination compris ing a frame, a plate cylinder journaled in the frame, plates mounted on the surface of the plate cylinder and having edges of the printing areas peripherally spaced 9 to define a gutter bounded by land surfaces on the plates, an impression cylinder journaled in the frame adjacent the plate cylinder and having a yieldable blanket mounted thereon, means for adjusting the distance between the axes of said plate and impression cylinders to produce impression advancement to create pressure between thern, drive means for rotating the cylinders precisely in unison, said impression cylinder having a segment removed from its surface to provide a gradual reduction in pressure followed by gradual increase in pres- 10 sure as gutter position is passed, the segment having a central portion of maximum thickness ranging between 25% and 75% of the impression advancement and a 10 width between 1.50 and 3.50 times the wave length of the observed streaking vibration.

References Cited UNITED STATES PATENTS 894,200 7/1908 Goss 101249 1,584,828 5/1926 Wood 101-219 3,303,779 2/1967 Giori 101229 FOREIGN PATENTS 542,586 1/1932 Germany.

WILLIAM B. PENN, Primary Examiner.

J. R. FISHER, Assistant Examiner. 

